Yeah, don't run a skimmer when the pond might be freezing.
Skimmer float weirs rely on being able to float freely to position themselves correctly. If ice interferes with their motion, you can end up with a waterfall that can trap fish.

If your pond is frozen over the fish will probably die. Then the dead fish will be sucked into the skimmer. Hydrogen disulfide will continue to be produced by decaying matter and will be trapped under the ice. Hydrogen disulfide is toxic in very small concentrations. Normally it will be released into the atmosphere, but it becomes trapped under the ice and lethal concentrations can form.
Either thaw the ice or cut it out to form breathing holes. Don't pound on the ice. Shock waves can rupture the fish's air bladder, killing the fish. I lost a pond of koi when the electricity went out for 12 hours and ice formed completely over the pond surface.

I have always had doubts as to the prevalent belief that fish will automatically die in a completely iced over pond. Naturally occurring ponds in the Northern latitudes are prone to completely ice over every winter, yet the fish remain alive. If they didn't, these same ponds would be completely devoid of any fish and we know that is not the case.
Hydrogen sulphide gas is formed by the action of heterotrophic bacteria which breakdown Hydrogen disulphide (an inorganic oil) under anaerobic conditions. This gas is formed ONLY in when there is a complete lack of Oxygen.

Not sure if I follow your post, its a skimmer box with a pump in it . . .

Sounds like what I expected. Either the skimmer is a box that hangs off the side of the liner, or you have a center skimmer (those are not too common).
Either "skim" the water by having the water cascade over a floating weir. When the pump is off, this weir should float freely to just above the water's surface. When the pump is turned on, the water level behind the weir drops, which causes the floating weir to drop. Eventually it is just low enough to let a thin ribbon of water flow over, and that generally stops fish from entering. Now if ice gets in there, it can hold the weir down, leaving enough water for a fish to swim though.

Skimmers should not be run during freezing conditions. Yes, shut that pump.

Quote:

Originally Posted by tadpole

I have always had doubts as to the prevalent belief that fish will automatically die in a completely iced over pond. Naturally occurring ponds . . .

Naturally occurring ponds have way less fish per gallon of water. That leaves much more available dissolved oxygen. A frozen over man made backyard pond is like stuffing an elevator full of people, and duct taping every seam shut. Eventually they'll run out of air.

Quote:

Originally Posted by jeffslawnservice

. . . should I shut off the pump and install an aerator or since its already frozen is that a no go.

Yes. But don't put the aerator in the skimmer box.
If you have a heater, try to melt a small hole over spot at least 2' deep, but not the absolute deepest part of the pond. Then toss in a sinking aerator stone into there.

The bubbles will usually keep a small area ice free, and even if they don't, so long as you keep bubbling air into the pond, it will find a way back out, and your fish will be fine.

Since the bubbles stir up the water, I prefer to not actually put my winter bubbler in the deepest part. I prefer to leave the deepest parts more still, as the ground will warm the deepest water a little, when you stir that up, you lose heat out the top.

Oh, and yes, hydrogen sulfide only forms in anaerobic conditions. This is possible under piles of leaves at the bottom (which is why you do not want to let leaves accumulate, but also why you need to be careful cleaning them out), but not in open water in a pond.

Naturally occurring ponds have way less fish per gallon of water. That leaves much more available dissolved oxygen. A frozen over man made backyard pond is like stuffing an elevator full of people, and duct taping every seam shut. Eventually they'll run out of air.

This analogy may have some merit if the pond already has a stocking density too high for the pond's capacity. I hear of just as many reports of Koi (and Goldfish) surviving the winter in an iced-over pond as I do of Koi (and Goldfish) mortality in ponds that provided venting in the ice.
Koi and Goldfish are known to survive in anoxic (low oxygen) conditions for extended periods under otherwise normal conditions. Cold water will retain considerably more Oxygen than warm water and fish metabolism is extremely low at low temps. Koi and Goldfish require very little Oxygen to survive in cold (near freezing) water. Koi because they are a warm water specie and can adapt to low Oxygen levels and Goldfish because they have the ability to store Oxygen in their muscle tissue when necessary. This has been proven in several scientific studies.
I would need to see more than anecdotal evidence that venting or aeration would be required in an iced over pond with the appropriate fish load.

This analogy may have some merit if the pond already has a stocking density too high for the pond's capacity. . . Cold water will retain considerably more Oxygen than warm water and fish metabolism is extremely low at low temps. . . I would need to see more than anecdotal evidence that venting or aeration would be required in an iced over pond with the appropriate fish load.

Couldn't agree more. In a clean pond with the appropriate fish load, and the appropriate depth to leave sufficient liquid water, a few days (perhaps even weeks) of ice is not a concern.

But how many ponds do you see people installing with too many fish, in a 2' deep hard plastic box store liner, with 6" of rotting leaves at the bottom?
Granted, in those situations, surviving through winter is the least of their problems.

Also, while cold water does retain more oxygen, and the fish's metabolism is extremely slow in the cold, the actual volume of dissolved oxygen is still staggeringly low, and can be used up. With everything just right, sure a week or so iced over shouldn't be a problem. Now if you are in a climate that ices over for months, I'd think about it.

Last winter on Long Island, ice would have maybe only closed things over for a few days. But in winters past, I've seen ice cover ponds for a month or more. I used to use that heated ring with the dome over it. But I've found that if I put it over a bubbler, I don't even need to plug the heat in to keep a hole in the ice. Do I need to do all this. Perhaps not, but it is cheap insurance.